Boolean Modifier cutting the wrong part
I've had a chance to look at the actual blend file, and I found geometry problems.  Let me show you how I found and fixed one.
I took your Cube cutter into edit mode in a local window.  (If you have a number pad on your keyboard the / character on the pad toggles that mode.)
Then I used the select menu to select interior faces

Turns out you've got one:

That's going to confuse the boolean operator, so delete it.  Use the X key and select &quot;Faces&quot;.
It still doesn't work because you have two objects you need to apply the Boolean to, &quot;Blade&quot; and &quot;Middle Knifu&quot;; so I added the boolean to each.

and it worked.
ASIDE: You have a lot of bad geometry in both the blade and Middle Knifu.  In addition to interior faces you have duplicated vertices and you have interior edges.
ORIGINAL ANSWER STARTS HERE:
Be sure that you apply the Boolean modifier to the cube that represents the blade and not the object you are using to 'cut' the hole.  Here's a crude example:

What you don't see in the example is the cylinder I used as the 'cutter' object.  If you look at my scene collection you'll see why:

When you select the blade and go to the modifiers panel, the Boolean modifier should look like this:

Notice that the modifier is applied to the blade and that the &quot;Object&quot; in the modifier is the cylinder.
Below the Object there's a 'Solver' box.  I have it set to Fast.
A common rookie mistake is to apply the modifer but leave the cylinder visible.  If you do this it looks like nothing happened:

There are a couple of ways to solve this.  For the example I simply made the Cylinder invisible:

If you do it this way, the cylinder will still show up in renders.  You have to also make it invisible to the camera:

Alternatively, if you are going to use a lot of Booleans a better technique is to move the cylinder to a new collection, usually called 'cutters':
Select the cylinder and type M.  this will bring up a menu:

Since you don't already have a collection, select &quot;new collection&quot; and give the collection a name.

Now all you have to do is uncheck the box for the cutters and the cylinder won't show up in the 3D view or on a render: